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Oil Furnace Fumes

Oil fired forced-air installation had the flue pipe jammed on the breech so far that the flue pipe extended into the fan plenum. Breech draft was -.04, presumably the fan plenum pressure would drop well below -.04 when the fan starts. This would cause flue gas to be sucked into the return air, and there was a plume of soot on the outside of the breech pipe that passes through the fan plenum.

I'd guess that a proper installation keeps that external flue pipe well outside of the fan plenum. About how far from the furnace wall do you typically keep the end of the external flue pipe?

those photos are almost worthless to show any detail!!! Maybe it is the host site. They are small.

I would all but bet that the flue does NOT extend into the plenum or the installers did one major hack job.

there are some units that the pipe runs in front of the closed off fan area, and this may be one of those.

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from an excerpt by Paul Jacob in Sun City, AZ

Only qualified PROs can perform this testing & setting of an oil fired furnace; this can be life threating to occupants if service work is not performed properly!

However, this is information that consumers need to know... for their ability to chose qualified technicians...to assure their families safety from CO poisoning, etc.

The PRO Technician needs good instruments to test the various draft aspects of an Oil furnace; after it has been operating for around 8 to 10 minutes to sufficiently get the walls of the combustion chamber white hot.

If any of the three fuel oil filters leading to the nozzle are partially plugged the furnace may NOT produce the required combustion heat to operate within the equipments' human safety requirements...

All these tests call for a well qualified Oil furnace PRO TECH knowledgeable Specialist.

The overfire test is performed first by the Tech drilling a 1/4 inch hole in the pressure relief door over the burner; often called the observation door.

The 1/4" probe pipe must be long enough to reach the inner edge of the combustion chamber; may need o use a copper tube extension.

Start with a barometric damper draft control setting of -0.04" of W.C.

The flow rate of the heat exchanger must be taken into account for proper burner operation. By measuring both the overfire draft & stack draft resistance on a new oil furnace, the design draft resistance can be determined.

The difference between the overfire draft & stack draft will be the heat exchanger draft flow resistance.

It is necessary to have a -0.06" stack draft to produce an overdraft of -0.04" of WC.

When checking the units performance if after a considerable period of operation, if the heat exchanger flow resistance has doubled,it is necessary to mechanically clean the heat exchanger flue passages.

A negative pressure must be maintained on the heat exchanger to prevent the products of combustion from being forced into the occupied area.

Thanks for the replies. I'm not sure what's happening to the photos, maybe they got crunched in the mobile upload.

Make/Model: Armstrong Air, LBR80C112/125D20-3A, installed about a year ago.

Maybe I'm using some words incorrectly. The term "flue pipe" always meant the sheet metal piping that connects the furnace to the chimney. There is a "breech pipe" (?), part of the furnace, welded to the heat exchanger, that passes through the fan compartment (plenum?), and stubs out the back a few inches to permit connection of the flue pipe.

In this case, the flue pipe was jammed so far onto the breech stub that it extended into the fan compartment. It's not a real tight fit, so the pressure difference could easily draw flue gases into the return air.

I wouldn't call this a hack job, the work is neat, but this little detail was overlooked for quite a while. How far does the flue pipe usually slide onto the breech stub? Surely it isn't supposed to extend the whole way into the furnace cabinet?

For safeties sake the furnace return air chamber ought to be sealed-off from the interior exhaust flu gas pipe. otherwise the return air chamber could also end-up sucking air into it from the basement.

That would have the appearance of faulty design, because over time, the flue pipe could rust through allowing flu gases to go directly into the RA stream & into the living areas of the home.

Many homes have a clothes washer in the basement, the soap & chlorine bleach chemicals will adhere to the interior of the heat exchanger, vent pipe & vent stack, eventually rusting through them. this could happen as soon as 5 to 10 years.

For example; if I had a flu pipe rusting badly; were it me; I'd simply replace the oil furnace with a new one or, with a propane furnace getting its combustion air from outdoors; Natural Gas if available, is a much less costly fuel with which to heat your home.

Pics are better, still not great. The second one appears to be taken looking up at the breech inside the blower compartment. The breech pipe looks a little distorted but it may be caused by the camera flash. Lowboy oil furnaces are available in a couple of different configurations, one being a front flue design, the other, a rear flue design. With the rear flue design, the breech pipe actually runs through the blower compartment. From what I can see of your intallation from the photos provided, nothing looks out of the ordinary to me. The end of the elbow does protude slightly past the end cabinet panel and it would not hurt to bump it back a bit. If the fitment is loose, some combustion by-products might possibly get drawn into the blower compartment. If the flue pipe is loose on the breech, a better solution might be to have the installer use a drawband to connect the elbow to the breech. In any case, all joints should be fastened with three (3) sheetmetal screws and any long pipe runs should be supported by metal strapping to prevent sagging or undue stress on the pipe and elbows. Under no circumstances should tape or sealant be applied to the the flue pipe or elbows. Termination temperatures of oil fired equipment exceed 450*F. Hope this helps.